The present invention relates to an electrophotographic image forming apparatus such as an electrophotographic printer, and in particular, to a diameter of a toner particle.
In conventional electrophotographic printers, in general, after an electrostatic latent image is formed on the surface of a photosensitive drum by means of a laser scanner, an LED head or the like, toner particles are attached to the electrostatic latent image in a developing section to form a toner image on the surface of the photosensitive drum. Then, the toner image is transferred onto paper as a transfer material in a transfer section.
The paper is then fed to a fixing device, and the toner image is fixed on the paper by the fixing device.
The average diameter of the toner particles is in a range of 5 .mu.m! to 15 .mu.m! and varies within the range of approximately 10 .mu.m!. The resolution of the electrophotographic printer is determined to be 300 dpi! or 600 dpi! in terms of the laser scanner, the LED head or the like, and an electrostatic latent image is to be formed with the resolution of 300 dpi! or 600 dpi!.
In this case, at least ten toner particles are to be attached to each of irradiated spots constituting the electrostatic latent image during the development.
In the conventional electrophotographic printers, in order to increase the resolution, it is necessary to use the toner having a small particle diameter. Nevertheless, if the toner with a small particle diameter is used, the ratio of the toner particles transferred onto the paper by the transfer process to all the toner particles constituting the toner image the surface of the photosensitive drum, that is, a transfer efficiency, will be reduced.
This happens because, since the toner particles with a smaller diameter are readily subject to the Van der Waals force when attached to the electrostatic latent image on the surface of the photosensitive drum, the sufficient amount of the toner particles on the surface of the photosensitive drum can not be attracted to the paper by the electrostatic forces in the transfer section.
Further, in order to have a sufficiently high print density of the toner image, it is necessary to render the thickness of a toner layer on the developing roller about twice as large as the diameter of the toner particle. However, when the toner particles with a small diameter are used, it is difficult to render the thickness of the toner layer uniform, leading to unevenness of the toner image.
Furthermore, in order to increase the resolution of the electrophotographic printer to 1200 dpi!, for example, it is necessary to use the toner particles with a diameter of not more than 5 .mu.m!. However, such toner particles tend to float in the air, thus resulting in the pollution of the environment.